What Are the Characteristics of a Sea Squirt?

Underwater scene of blue tunicates (Rhopalaea crassa), Sulawesi, Indonesia
Panoramic Images/Panoramic Images/Getty Images

A sea squirt may look more like a vegetable, but it is an animal. Sea squirts are more scientifically known as tunicates or ascidians, as they belong to the Class Ascidiacea. Surprisingly, these animals are in the same phylum we are - Phylum Chordata, which is the same phyla that include humans, whales, sharks, pinnipeds, and fish. 

There are over 2,000 species of sea squirts, and they are found throughout the world. Some species are solitary, while some form large colonies.

Characteristics of Sea Squirts

Sea squirts have a tunic, or test, which attaches to a substrate 

Sea squirts have two siphons - an inhalant siphon, which they use to pull water into their body, and an exhalant siphon, which they use to expel water and wastes. When disturbed, a sea squirt may eject water from its siphon, which is how this creature got its name. If you remove a sea squirt from the water, you may get a wet surprise!

Sea squirts eat by taking in water through their inhalant (incurrent) siphon. Cilia create a current that passes the water through the pharynx, where a layer of mucus traps plankton and other small particles. These are then passed into the stomach, where they are digested. The water carries waste out through the intestines and is expelled via the exhalant (excurrent) siphon. 

Sea Squirt Classification

  • Kingdom: Animalia
  • Phylum: Chordata
  • Subphylum: Urochordata
  • Class: Ascidiacea

Because sea squirts are in the phylum Chordata, they are related to vertebrates such as humans, whales, and fish. All chordates have a notochord or primitive backbone at some stage. In sea squirts, the notochord is present in the animal's larval stage.

Where Do Sea Squirts Live?

Sea squirts attach to things like piers, docks, boat hulls, rocks, and shells, many in subtidal locations. They may attach singly or in colonies. 

Sea Squirt Reproduction

In addition to eating, the inhalant siphon is used for reproduction. Most sea squirts are hermaphroditic, and while they produce both eggs and sperm, the eggs stay inside the tunicate's body and are fertilized by sperm that enters the body through the inhalant siphon. The resulting larvae look like a tadpole. This tadpole-like creature soon settles to the ocean bottom or to a hard substrate, where it attaches to life and secretes a leathery, cellulose-based substance that makes up the tunic that encases it. The resulting animal is barrel-shaped. 

Sea Squirts may also reproduce asexually by budding, in which a new animal splits off or grows out of the original animal. This is how colonies of sea squirts form.

References and Further Information

  • Coulombe, D.A. 1984. The Seaside Naturalist. Simon & Schuster. 246pp.
  • Meinkoth, N.A. 1981. National Audubon Society Field Guide to North American Seashore Creatures. Alfred A. Knopf: New York.
  • Newberry, T. and R. Grossberg. 2007. "Tunicates." In Denny, M.W., and S.D. Gaines, eds. Encyclopedia of Tidepools and Rocky Shores. University of California Press. 705pp.